Layered products in which a gas barrier layer containing aluminum or aluminum oxide as a component is formed on a plastic film have been conventionally well-known. Such layered products are used as packaging materials for protecting articles (such as foods) which are susceptible to quality change induced by oxygen. In many cases, such a gas barrier layer is formed on a plastic film by a dry process such as physical or chemical vapor deposition. Aluminum-deposited films have light shielding properties as well as gas barrier properties and are typically used as packaging materials for dry foods. Aluminum oxide-deposited films, which have transparency, are characterized by allowing visual recognition of contained substances and by enabling check for foreign matters with a metal detector and heating with a microwave oven. These films are thus used as packaging materials in a wide variety of applications such as retort food packaging.
For example, Patent Literature 1 discloses a gas barrier layer containing aluminum, the gas barrier layer being a transparent gas barrier layer composed of aluminum atoms, oxygen atoms, and sulfur atoms. Patent Literature 1 discloses a method for forming the transparent gas barrier layer by reactive sputtering.
Patent Literature 2 discloses a transparent gas barrier layer composed of a reaction product of aluminum oxide particles and a phosphorus compound. Patent Literature 2 discloses a method for forming the gas barrier layer, in which a coating liquid containing aluminum oxide particles and a phosphorus compound is applied onto a plastic film, then dried and heat-treated.
When the layered product is used as a packaging material, a laminated film composed of the layered product and polyethylene laminated thereon with an adhesive is usually used to impart additional properties such as heat-sealing properties. An example of the method for producing such a laminated film is extrusion coating lamination in which an adhesive is applied to one surface of the multilayer structure using a coater and molten polyethylene is laminated on the adhesive.
However, the present inventors have found that when the layered product is subjected to extrusion coating lamination, the gas barrier properties of the resulting multilayer structure may deteriorate. There has thus been a demand for a multilayer structure capable of maintaining high barrier properties even after being formed by extrusion coating lamination.